Acousto-optical systems

ABSTRACT

An acousto-optical system which diffracts or modulates light passing through a crystal of Tl3AsSe3 by means of sound waves in the crystal is disclosed. Sound waves are generated in the crystal by means of a transducer connected to an RF generator. The acousto-optical system may be in the form of a display device, a laser modulator, or other devices. An acoustic delay line using a crystal of Tl3AsSe3 is also disclosed.

United States Patent [191 Feichtner et al.

[ ACOUSTO-OPTICAL SYSTEMS [76] Inventors: John D. Feichtner, 4725 Millstream Dr., Murrysville, Pa. 15608; Milton Gottlieb, 2310 Marbury Rd., Pittsburgh, Pa. 15221; George W. Roland, 1176 Bucknell Dr., Monroeville, Pa. 15146 22 Filed: Apr. 17, 1973 21 Appl. No.: 351,919

Related US. Application Data [62] Division of Ser. No. 209,849, Dec. 20, 1971, Pat. No.

52 us. c1. 333/30 R, 333/72 51 Int. Cl. H03h 7/30, H03h 9/30 58 Field of Search 333/30 R, 72; 350/161;

[451 Apr. 16, 1974 [56] References Cited UNITED STATES PATENTS 3,509,489 4/1970 Burns et al 330/55 X Primary 'ExaminerJames W. Lawrence. Assistant Examiner-Marvin Nussbaum Attorney, Agennor Firm-R. D. Fuerle 5 7] ABSTRACT 3 Claims, 4 Drawing Figures GENERATOR PATENTEDAPR 16 I974 GENERATOR GENERATOR RF GENERATOR FIGS RF GENERATOR 3| FIG.4

GENERATOR ACOUSTO-OPTICAL SYSTEMS This is a division, of application Ser. No. 209,849 filed Dec. 20, 1971, now Pat. No. 3,746,886, issued July 17, 1973.

BACKGROUND OF THE INVENTION In 1932 Brillouin discovered that high frequency sound waves can cause diffraction of light. With the advent of the laser and advances in high frequency acous-, tic techniques many applications for this phenomenon have been found such as display devices or lasermodulators.

A sound wave moving in crystal is composed of alternating compression and rarefaction fronts. The indices of refraction in these fronts is different, so that the crystal acts as a diffraction grating, diffracting light which passes through it, the angle of diffraction increasing as the frequency of the sound wave increases, and the amount of light diffracted increasing with the intensity.

of the sound wave.

There are 'two modes of diffraction, the Debye-Sears mode and the Bragg mode. The Debye-Sears mode is obtained if the width of the acoustic'beam is less than about A/(4}\) and the Bragg mode is obtained if the width of the acoustic beam is greater than about [t /(4A) where A is the acoustic wavelength and )t is the light wavelength. In both modes the acoustic wavelength A must be greater than the light wavelength A, and A must be within the transparency region of the crystal. In the Debye-Sears mode light enters the crystal parallel to the acoustic wave fronts diffracting angle) and is multiply-diffracted into many images or orders of the initial light beam. In the Bragg mode light enters the crystal at the Bragg angle (ii to the acoustic wavefronts where sin (1) AM. In this mode theacous tic wavelength and the Bragg angle are matched to the particular light'wavelength, and a single image is diffracted from the crystal at the Bragg angle (1) to the acoustic wave fronts. v i

A good acousto-optical material should have a high figure of merit, M a measurement of the amount of light diffracted for a given amount of acoustic power, where M np/pv and n is the refractive index, p is the photoelastic coefficient, p is the density, and v is the acoustic velocity. As the formula indicates, a low velocity will give a high figure of 'merit and, in addition,

it'will give a greater delay per unit length if the crystal is used in a delay line thus'ipermitting acoustic signal processing devices to have smaller physical dimensions. A good acousto-optical material should also have a low acoustic attenuation, allowing a high frequency wave to propagate a long distance before it is absorbed.

SUMMARY OF THE INVENTION than prior art materials, they have a figure of merit higher than most prior art materials, and their acoustic attenuation is low. They-are used in acousto-optical systems such as display devices and laser modulators,

' and are also used in acoustic delay lines as herein describedw FIG. 1 is an isometric diagrammatic drawing of a display device.

FIG. 2 is a diagrammatic drawing of a laser modulator of the internal configuration.

FIG. 3 is a diagrammatic drawing of a laser modulator of the external configuration.

FIG. 4 is a diagrammatic drawing of an acoustic delay line.

THE CRYSTAL A detailed description of the preparation of a crystal of Tl AsSe is given in application Ser. No. 181,409 by George W. Roland and John D. Feichtner titled, Non- Linear Optical Compound, Crystal, and Devices, filed Sept. 17, 1971. Briefly, the compound Tl AsSe is pre- The crystal of this invention is negative uniaxial and piezoelectric. Its Laue class is 327m, and the diffraction aspect derived from X-ray data is R**. Of the two permitted space groups, R3m and R32, space group R3m is more probable basedonthe observed absence of optical activity. The length the a axis is about 9.90

and the c axis about 7.1.3 and the transparency region is about 1.25 to about 17pm. Its density is 7.83 gms/cc.

The acousto-opticalfigure of merit, M is 995 at )t= 3.39am (measured by a direct comparison method with fused silica at )t=0.6328p.m using the p 33 component of the photoelastic tensor, i.e. sound wave pr'opagated along the c axis, light polarized perpendicular to the c axis and propagated normal to the sound wave). From this figure of merit p 33 was estimated to be 0.228. Generally, the Debye-Sears mode is obtained at acoustic frequencies less than about 50M1-1z (megahertz) and the Bragg mode at acoustic frequencies greater than about SOMHz.

The following table gives the measured refractive indices of a Tl AsSe crystal where n is the refractive index of an ordinary wave and n,, is the refractive index of an extroaordinary wave.

Wavelength Uncertainty in n It, Mi Mu The width of the crystal is not critical and several millimeters are typical; however, the optical faces of the crystal should be wider than a laser can be focused, about 10 cm., so that light is not wasted. The crystal should not be too thin in the direction of light propaga tion as this will result in poor interaction between the light and sound and therefore a low intensity defraction, and if the crystalis too thick light loss due to absorption will be high. v

THE SOUND w vEs [Te sound wave maybe a longitudinal waye, vi i 1% the particle motion is in the direction of propagation of the wave, or it may be a shear wave, where the particle motion is perpendicular to the propagation direction of the wave, or it may be a combination of both. Preferably, it is either pure shear or pure longitudinal because the two waves travel at different velocities and quickly become out of phase. For delay line applications shear waves are desirable because of their lower velocity. Pure shear waves are obtained by propagating the wave in a pure shear direction (found experimentally by orientating the crystal) using a shear wave generating transducer such as Y cut or A-Ccut quartz, which is glued to the crystal. Longitudinal waves are obtained by propagating the wave along the c-axis or another pure longitudinal direction using a longitudinal wave generating transducer such as X-cut quartz which is glued to the crystal. I

At 3OMHz, the velocity of shear waves propagating parallel to the c-axis is 1.03 X 10 cm/sec. For propagation along one of the a-axes, the velocity of longitudinal waves is 1.98 X cm/sec. and of shear waves is 9.9 X 10 cm/sec. I

DISPLAY DEVICES In a display device a light beam is directed at the crystal and the deflected beam which leaves the crystal is directed at some type of viewing screen.

In FIG. 1 RF generators l and 2 send RF signals to transducers 3 and 4 respectively which respectively generate vertically moving and horizontally moving sound waves in crystal 5, preferably in the Bragg mode so that there is only onediffracted beam. The light, which is preferably parallel and polarized for good resolution, is obtained from laser 6 which generates a coherent beam of light 7 directed at one of the two parallel optical faces 8 of crystal 5. Light passing through crystal 5 is directed at various spots 9 on viewing screen 10 by means of the vertically and horizontally moving sound waves generated by transducers 3 and 4. Lens 1 1 focuses the light at the spot.

The illuminated spots may each be a page of information which is then optically enlarged and projected on a second viewing screen (not shown). The illuminated spots could also in themselves form a pattern. For example, viewing screen 10 could be a infrared-sensitive phosphor coated screen such as zinc sulfide doped with lead and copper and flooded with UV light and the successive illumination of selected spots would form a picture similar to a TV picture. Or, viewing screen 10 could be infrared or thermally-quenched UV-excited phosphor screen where ultraviolet light causes the entire screen to be illuminated, but each selected spot successively struck by the beam from crystal 5 is darkened to form a picture on the screen.

LASER MODULATOR In a laser modulator the acousto-optical system modulates a portion of the output vof the lasing medium. If the light is focused to less than about 10 or 10 cm. it will be modulated but not diffracted. For greater diameter focal spots it will be both diffracted and modulated. A laser modulator could be used, for example, to

send signals by means of the fluctuating laser beam intensity.

FIG. 2 shows a laser modulator of the internal configuration. in FIG. 2, lasing medium 12 produces a beam of coherent light which is multiply-reflected between mirrors 13 and 14. Mirror 13 totally reflects the light and mirror 14 partially reflects it and partially transmits it as the laser output 32. Interposed between lasing medium l2 and mirror 14 is a crystal 15 of Tl AsSe (The crystal could also be positioned between mirror 13 and the lasing medium). To crystal 15 is affixed a transducer 16 which is electrically connected to an RF generator 17. This generator produces a radio-frequency electrical signal which transducer 16 converts into an acoustic wave which moves through crystal 15 diffract- 'ing light as shown at 18.

ACOUSTIC DELAY LINE An acoustic delay line causes an electrical signal to be delayed for the length of time required for an acoustic signal to traverse the crystal, L/V, where L is the length of the crystal and V is the acoustic velocity. Unlike many other methods of delaying an electricalsignal, an acoustic delay line preserves the original configuration of the signal.

In FIG. 4, RF generator 28 provides the electrical signal-to be delayed. This signal is electrically transmitted to transducer 29 which converts the signal to an acoustic wave which is propagated through crystal 30 of Tl Asse At the other end of the crystal transducer 31 detects the acoustic wave and converts it into an electrical signal.

We claim:

1. In an acoustic delay line having 1. a sound-transmitting medium;

2. means for generating sound waves in said medium;

and

3. means for detecting said sound waves after they have traversed said medium, the improvement which comprises a medium which consists of a crystal of Tl AsSe greater than lcm in volume.

2. An acoustic delay line according to claim 1 wherein said means for generating sound waves in said crystal is a first transducer affixed to said crystal and said means for detecting said sound waves is a second transducer affixed to said crystal opposite said first transducer.

3. An acoustic delay line according to claim 1 wherein said sound waves are shear waves.

t l l k 

2. means for generating sound waves in said medium; and
 2. An acoustic delay line according to claim 1 wherein said means for generating sound waves in said crystal is a first transducer affixed to said crystal and said means for detecting said sound waves is a second transducer affixed to said crystal opposite said first transducer.
 3. An acoustic delay line according to claim 1 wherein said sound waves are shear waves.
 3. means for detecting said sound waves after they have traversed said medium, the improvement which comprises a medium which consists of a crystal of Tl3AsSe3 greater than 1cm3 in volume. 